Waveguide optical element and its driving method

Abstract

A waveguide optical element includes a waveguide layer which is formed on a substrate, through which light propagates and which has an end face from which the propagation light becomes incident on the waveguide layer or emerges, a thin film formed on the end face of the waveguide layer, for increasing or decreasing a reflectance of light at end face, and control means having an electrode formed on a portion of the waveguide layer, for controlling a refractive index of the portion of the waveguide layer, which portion is in contact with the thin film. And its driving method is provided.

What is claimed is: 
     
       1. A waveguide optical element comprising: a substrate;   a waveguide layer which is formed on said substrate, and through which light propagates, said waveguide layer having an end face from which the propagation light becomes incident on said waveguide layer, or emerges;   a thin film, formed on the end face of said waveguide layer, for increasing or decreasing a reflectance of light at the end face; and   control means for changing a refractive index of a portion of said waveguide layer, which portion is contiguous with said thin film to vary a reflectance of light at the end face of said waveguide layer.   
     
     
       2. An element according to claim 1, wherein said control means comprises an electrode formed on the portion of said waveguide layer, which portion is contiguous with said thin film. 
     
     
       3. An element according to claim 1, further comprising: a first cladding layer formed between said substrate and said waveguide layer; and   a second cladding layer formed to be contiguous with a surface of said waveguide layer, which surface is opposite to a surface thereof facing said first cladding layer.   
     
     
       4. A semiconductor optical amplifier comprising: a substrate;   a semiconductor waveguide layer which is formed on said substrate, and through which light propagates, said waveguide layer having a pair of end faces constituting a laser resonator;   amplifier means for injecting a current into said waveguide layer to amplify the propagation light;   antireflection films formed on the two end faces of said waveguide layer; and   control means for changing refractive indices of portions of said waveguide layer, which portions are contiguous with said antireflection films to vary a reflectance of light at the end faces of said waveguide layer.   
     
     
       5. An amplifier according to claim 4, wherein said control means comprises first electrodes formed on the portions of said waveguide layers, which portions are contiguous with said antireflection films. 
     
     
       6. An amplifier according to claim 5, wherein said amplifier means comprises a second electrode formed on a portion, other than the portions contacting said antireflection films, of said waveguide layer independently of said first electrodes. 
     
     
       7. An amplifier according to claim 6, further comprising an insulating film for restricting a region where said first and second electrodes electrically contact said waveguide layer to a stripe-shaped region extending in a resonance direction of said resonator. 
     
     
       8. An amplifier according to claim 4, wherein said waveguide layer is formed into a stripe pattern extending in a resonance direction of said resonator on a portion of said substrate, and a buried layer is formed on the remaining portion of said substrate. 
     
     
       9. An amplifier according to claim 4, further comprising: a first cladding layer formed between said substrate and said waveguide layer; and   a second cladding layer formed to be contiguous a surface of said waveguide layer, which surface is opposite to a surface thereof facing said first cladding layer.   
     
     
       10. An amplifier according to claim 9, wherein said first cladding layer comprises an n-type semiconductor, said second cladding layer comprises a p-type semiconductor, and said waveguide layer comprises a non-doped semiconductor. 
     
     
       11. A method of driving a semiconductor optical amplifier which comprises a substrate, a semiconductor waveguide layer which is formed on said substrate, and through which light propagates, said waveguide layer having a pair of end faces constituting a laser resonator, amplifier means for injecting a current into said waveguide layer to amplify the propagation light, antireflection films formed on the two end faces of said waveguide layer, and control means for controlling refractive indices of portions,. of said waveguide layer, which portions are contiguous with said antireflection films, comprising the steps of: inputting light from one end face of said waveguide layer;   causing said amplifier means to inject a current into said waveguide layer so as to amplify the input light;   detecting the amplified light emerging from the other end face of said waveguide layer; and   causing said control means to control refractive indices of the portions of said waveguide layer, which portions are contiguous with said antireflection films, in accordance with an intensity of the detected light.   
     
     
       12. A method according to claim 11, wherein said control means controls the refractive indices of the portions of said waveguide layer, which portions are contiguous with said antireflection films, so as to maximize an intensity of the amplified light without an intensity of a light component emitted from said optical amplifier itself of the detected light. 
     
     
       13. A method according to claim 11, wherein said control means controls the refractive indices by injecting a current into the portions of said waveguide layer, which portions are contiguous with said antireflection films. 
     
     
       14. A method according to claim 11, wherein the portions of said waveguide layer, which portions are contiguous with said antireflection films have a quantum well structure, and said control means applies a voltage to the portions to control the refractive indices thereof. 
     
     
       15. A method of driving a semiconductor optical amplifier which comprises a substrate, a semiconductor waveguide layer which is formed on said substrate, and through which light propagates, said waveguide layer having a pair of end faces constituting a laser resonator, amplifier means for injecting a current into said waveguide layer to amplify the propagation light, antireflection films formed on the two end faces of said waveguide layer, and control means for controlling refractive indices of portions of said waveguide layer, which portions are contiguous with said antireflection films, comprising the steps of: inputting light modulated at a first frequency from one end face of said waveguide layer;   causing said amplifier means to inject a current modulated at a second frequency into said waveguide layer so as to amplify the input light;   detecting the amplified light emerging from the other end face of said waveguide layer as an electrical signal;   separating the detected electrical signal into a first signal component having the first frequency, and a second signal component having the second frequency; and   causing said control means to control refractive indices of the portions of said waveguide layer, which portions are contiguous with said antireflection films, so as to maximize the first signal component without increasing the second signal component of the separated signal components.   
     
     
       16. A method according to claim 15, wherein said control means controls the refractive indices by injecting a current into the portions of said waveguide layer, which portions are contiguous with said antireflection films. 
     
     
       17. A method according to claim 15, wherein the portions of said waveguide layer, which portions are contiguous with said antireflection films have a quantum well structure, and said control means applies a voltage to the portions to control the refractive indices thereof. 
     
     
       18. An optical amplifier apparatus comprising: a semiconductor optical amplifier, which comprises a substrate,   a semiconductor waveguide layer which is formed on said substrate, and through which light propagates, said waveguide layer having a pair of end faces constituting a laser resonator,   amplifier means for injecting a current into said waveguide layer to amplify the propagation light,   antireflection films formed on the two end faces of said waveguide layer, and   control means for controlling refractive indices of portions of said waveguide layer, which portions are contiguous with said antireflection films;     a photodetector for detecting some components of the light amplified by said optical amplifier; and   a control circuit for driving said control means of said optical amplifier in accordance with an output from said photodetector.   
     
     
       19. An apparatus according to claim 18, wherein said control means of said optical amplifier comprises first electrodes formed on the portions of said waveguide layers, which portions are contiguous with said antireflection films. 
     
     
       20. An apparatus according to claim 19, wherein said amplifier means of said optical amplifier comprises a second electrode formed on a portion, other than the portions contiguous with said antireflection films, of said waveguide layer independently of said first electrodes. 
     
     
       21. An apparatus according to claim 20, wherein said optical amplifier further comprises an insulating film for restricting a region where said first and second electrodes electrically contact said waveguide layer to a stripe-shaped region extending in a resonance direction of said resonator. 
     
     
       22. An apparatus according to claim 18, wherein said waveguide layer is formed into a stripe pattern extending in a resonance direction of said resonator on a portion of said substrate, and a buried layer is formed on the remaining portion of said substrate. 
     
     
       23. An apparatus according to claim 18, wherein said optical amplifier further comprises: a first cladding layer formed between said substrate and said waveguide layer; and   a second cladding layer formed to be contiguous with a surface of said waveguide layer, which surface is opposite to a surface thereof facing said first cladding layer.   
     
     
       24. An apparatus according to claim 23, wherein said first cladding layer comprises an n-type semiconductor, said second cladding layer comprises a p-type semiconductor, and said waveguide layer comprises a non-doped semiconductor. 
     
     
       25. A semiconductor optical amplifier comprising: a substrate;   a semiconductor waveguide layer which is formed on said substrate, and through which light propagates, said waveguide layer having a pair of end faces constituting a laser resonator, refractive index control portions contiguous with the end faces, and an active portion;   antireflection films formed on the two end faces of said waveguide layer;   a first electrode, formed on said active portion of said waveguide layer, for injecting a current into said active portion to amplify the propagation light; and   second electrodes, formed on said refractive index control portions of said waveguide layer, for changing refractive indices of said refractive index control portions to vary a reflectance of light at the end of faces.   
     
     
       26. An amplifier according to claim 25, further comprising an insulating film for restricting a region where said first and second electrodes electrically contact said waveguide layer to a stripe-shaped region extending in a resonance direction of said resonator. 
     
     
       27. An amplifier according to claim 25, wherein said waveguide layer is formed into a stripe pattern extending in a resonance direction of said resonator on a portion of said substrate, and a buried layer is formed on the remaining portion of said substrate. 
     
     
       28. An amplifier according to claim 25, further comprising: a first cladding layer formed between said substrate and said waveguide layer; and   a second cladding layer formed to be in contact with a surface of said waveguide layer, which surface is opposite to a surface thereof facing said first cladding layer.   
     
     
       29. An amplifier according to claim 28, wherein said first cladding layer comprises an n-type semiconductor, said second cladding layer comprises a p-type semiconductor, and said waveguide layer comprises a non-doped semiconductor. 
     
     
       30. An amplifier according to claim 25, wherein each of said refractive index control portions of said waveguide layer comprises a quantum well structure obtained by alternately stacking well layers and barrier layers. 
     
     
       31. An amplifier according to claim 30, wherein said well layers and said active portion of said waveguide layer are formed of GaAs, and said barrier layers are formed of AlGaAs. 
     
     
       32. An amplifier according to claim 25, wherein said active portion of said waveguide layer is formed of GaAs, and said refractive index control portions of said waveguide layer are formed of AlGaAs.